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Kinematic Pile-Soil Interaction in Liquefied and Nonliquefied Ground


In Part I of this dissertation, equivalent static analysis (ESA) procedures for computing foundation demands during lateral spreading are applied to two parallel bridges that were damaged during the 2010 M 7.2 El Mayor-Cucapah earthquake in Baja California, Mexico. A railroad bridge span collapsed, whereas the adjacent highway bridge survived with one support pier near the river having modest flexural cracking of cover concrete. Cone penetration and geophysical test results are presented along with geotechnical and structural conditions evaluated from design documents. ESA using a beam-on-Winkler foundation model is found to accurately predict observed responses when liquefaction-compatible inertia demands are represented as spectral displacements that account for resistance from other bridge components. Pier columns for the surviving bridge effectively resisted lateral spreading demands in part because of restraint provided by the superstructure. Collapse of the surviving bridge is incorrectly predicted when inertial demands are computed for the individual bent in isolation from other components, and are represented by forces that do not consider global restraint.

In Part II, results of a parametric study of the influence of kinematic pile-soil interaction on foundation-input motions (FIM) are presented. One-dimensional nonlinear ground response analysis was used to define free-field motions, which were subsequently imposed on a beam-on-nonlinear-dynamic-Winkler-foundation pile model. The free-field ground surface motion (FFM) and top-of-pile “foundation-input motion” (FIM) computed from these results were then used to compute transfer functions and spectral ratios for use with the substructure method of seismic analysis. A total of 1,920 parametric combinations of different pile sizes, soil profiles, and ground motions were analyzed.

Results of the study show that significant reductions of the FFM occur for stiff piles in soft soil, which could result in a favorable reduction in design demands for short-period structures. Group effects considering spatially-variable (incoherent) ground motions are found to be modest over the footprint of a typical bridge bent, resulting in an additional reduction of FFM by 10 percent or less compared to an equivalent single pile. This study aims to overcome limitations of idealistic assumptions that have been employed in previous studies such as linear-elastic material behavior, drastically simplified stratigraphy, and harmonic oscillations in lieu of real ground motions. In order to capture the important influence of more realistic conditions such as material nonlinearity, subsurface heterogeneity, and variable frequency-content ground motions, a set of models for predicting transfer functions and spectral ratios has been developed through statistical regression of the results from this parametric study. These allow foundation engineers to predict kinematic pile-soil interaction effects without performing dynamic pile analyses.

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